Minimizing Interfacial Recombination in 1.8 eV Triple-Halide Perovskites for 27.5% Efficient All-Perovskite Tandems.

All-perovskite tandem solar cells show great potential to enable the highest performance at reasonable costs for a viable market entry in the near future. In particular, wide-bandgap (WBG) perovskites with higher open-circuit voltage (VOC ) are essential to further improve the tandem solar cells' performance. Here, a new 1.8 eV bandgap triple-halide perovskite composition in conjunction with a piperazinium iodide (PI) surface treatment is developed. With structural analysis, it is found that the PI modifies the surface through a reduction of excess lead iodide in the perovskite and additionally penetrates the bulk. Constant light-induced magneto-transport measurements are applied to separately resolve charge carrier properties of electrons and holes. These measurements reveal a reduced deep trap state density, and improved steady-state carrier lifetime (factor 2.6) and diffusion lengths (factor 1.6). As a result, WBG PSCs achieve 1.36 V VOC , reaching 90% of the radiative limit. Combined with a 1.26 eV narrow bandgap (NBG) perovskite with a rubidium iodide additive, this enables a tandem cell with a certified scan efficiency of 27.5%.

Slot-Die Coated Triple-Halide Perovskites for Efficient and Scalable Perovskite/Silicon Tandem Solar Cells.

Wide bandgap halide perovskite materials show promising potential to pair with silicon bottom cells. To date, most efficient wide bandgap perovskites layers are fabricated by spin-coating, which is difficult to scale up. Here, we report on slot-die coating for an efficient, 1.68 eV wide bandgap triple-halide (3halide) perovskite absorber, (Cs0.22FA0.78)Pb(I0.85Br0.15)3 + 5 mol % MAPbCl3. A suitable solvent system is designed specifically for the slot-die coating technique. We demonstrate that our fabrication route is suitable for tandem solar cells without phase segregation. The slot-die coated wet halide perovskite is dried by a "nitrogen (N2)-knife" with high reproducibility and avoiding antisolvents. We explore varying annealing conditions and identify parameters allowing crystallization of the perovskite film into large grains reducing charge collection losses and enabling higher current density. At 150 °C, an optimized trade-off between crystallization and the PbI2 aggregates on the film's top surface is found. Thus, we improve the cell stability and performance of both single-junction cells and tandems. Combining the 3halide top cells with a 120 µm thin saw damage etched commercial Czochralski industrial wafer, a 2-terminal monolithic tandem solar cell with a PCE of 25.2% on a 1 cm2 active area is demonstrated with fully scalable processes.

Nano-optical designs for high-efficiency monolithic perovskite-silicon tandem solar cells.

Perovskite-silicon tandem solar cells offer the possibility of overcoming the power conversion efficiency limit of conventional silicon solar cells. Various textured tandem devices have been presented aiming at improved optical performance, but optimizing film growth on surface-textured wafers remains challenging. Here we present perovskite-silicon tandem solar cells with periodic nanotextures that offer various advantages without compromising the material quality of solution-processed perovskite layers. We show a reduction in reflection losses in comparison to planar tandems, with the new devices being less sensitive to deviations from optimum layer thicknesses. The nanotextures also enable a greatly increased fabrication yield from 50% to 95%. Moreover, the open-circuit voltage is improved by 15 mV due to the enhanced optoelectronic properties of the perovskite top cell. Our optically advanced rear reflector with a dielectric buffer layer results in reduced parasitic absorption at near-infrared wavelengths. As a result, we demonstrate a certified power conversion efficiency of 29.80%.

Co-Evaporated p-i-n Perovskite Solar Cells beyond 20% Efficiency: Impact of Substrate Temperature and Hole-Transport Layer.

For methylammonium lead iodide perovskite solar cells prepared by co-evaporation, power conversion efficiencies of over 20% have been already demonstrated, however, so far, only in n-i-p configuration. Currently, the overall major challenges are the complex evaporation characteristics of organic precursors that strongly depend on the underlying charge selective contacts and the insufficient reproducibility of the co-evaporation process. To ensure a reliable co-evaporation process, it is important to identify the impact of different parameters in order to develop a more detailed understanding. In this work, we study the influence of the substrate temperature, underlying hole-transport layer (polymer PTAA versus self-assembling monolayer molecule MeO-2PACz), and perovskite precursor ratio on the morphology, composition, and performance of co-evaporated p-i-n perovskite solar cells. We first analyze the evaporation of pure precursor materials and show that the adhesion of methylammonium iodide (MAI) is significantly reduced with increased substrate temperature, while it remains almost unaffected for lead iodide (PbI2). This substrate temperature-dependent evaporation behavior of MAI is also transferred to the co-evaporation process and can directly influence the perovskite composition. We demonstrate that the optimal substrate temperature window for perovskite deposition is close to room temperature. At high temperature, not enough MAI for precise stoichiometry is incorporated even with very high MAI rates. While, at temperatures below -25 °C, the conversion of MAI with PbI2 is inhibited, and an amorphous yet unreacted film is formed. We observe that perovskite composition and morphology vary widely between the organic hole-transport layers (HTLs) PTAA and MeO-2PACz. For all substrate temperatures, MeO-2PACz enables higher solar cell PCEs than PTAA. Through the combination of vapor-deposited perovskites and a self-assembled monolayer, we achieve a stabilized power conversion efficiency of 20.6%, which is the first reported PCE above 20% for evaporated perovskite solar cells in p-i-n architecture.

Mixtures of Dopant-Free Spiro-OMeTAD and Water-Free PEDOT as a Passivating Hole Contact in Perovskite Solar Cells.

Doped spiro-OMeTAD at present is the most commonly used hole transport material (HTM) in n-i-p-type perovskite solar cells, enabling high efficiencies around 22%. However, the required dopants were shown to induce nonradiative recombination of charge carriers and foster degradation of the solar cell. Here, in a novel approach, highly conductive and inexpensive water-free poly(3,4-ethylenedioxythiophene) (PEDOT) is used to replace these dopants. The resulting spiro-OMeTAD/PEDOT (SpiDOT) mixed films achieve higher lateral conductivities than layers of doped spiro-OMeTAD. Furthermore, combined transient and steady-state photoluminescence studies reveal a passivating effect of PEDOT, suppressing nonradiative recombination losses at the perovskite/HTM interface. This enables excellent quasi-Fermi level splitting values of up to 1.24 eV in perovskite/SpiDOT layer stacks and high open-circuit voltages ( VOC) up to 1.19 V in complete solar cells. Increasing the amount of dopant-free spiro-OMeTAD in SpiDOT layers is shown to enhance hole extraction and thereby improves the fill factor in solar cells. As a consequence, stabilized efficiencies up to 18.7% are realized, exceeding cells with doped spiro-OMeTAD as a HTM in this study. Moreover, to the best of our knowledge, these results mark the lowest nonradiative recombination loss in the VOC (140 mV with respect to the Shockley-Queisser limit) and highest efficiency reported so far for perovskite solar cells using PEDOT as a HTM.